1. Field of the Invention
The present invention relates to a protective film having a coat layer for a polarizing plate for the purpose of realizing a low water vapor transmission rate, a polarizing plate using the protective film and a liquid crystal display device using the polarizing plate.
2. Description of the Related Art
Liquid crystal display devices (hereinafter referred to simply as LCDs) have recently been replacing CRTs for their thinness, lightness and low power consuming. There is an increasing demand for polarizing plates along with propagation of LCDs. The field of application of LCDs is also expanding from conventional small products such as desktop electric calculators to larger products including automobile instruments, PC monitors and television sets.
A polarizing plate is generally formed by bonding a protective film for a polarizing plate (hereinafter referred to as protective film or protective layer) to one or both of the opposite surfaces of a polarizer having a polarizing ability, with an adhesive layer interposed between the polarizing plate and the polarizer.
Polyvinyl alcohol (also referred to as PVA hereinafter) is mainly used as material of a polarizer. A polarizer is formed bymonoaxial stretching of a PVA film followed by dying with iodine or bi-color dye (alternatively by first dying the PVA film with such a dye followed by monoaxial stretching), and then allowing the stretched PVA film to crosslink by means of a boron compound.
Cellulose triacetate (also referred to as TAC hereinafter) is mainly used to form the protective film because it is optically transparent and exhibits less birefringence, has a smooth surface, and offers excellent adhesion relative to a polarizer made of PVA by saponification treatment.
When cellulose triacetate is used for the protective layer, the image being displayed can become uneven after a long use due to changes in the size of the polarizer caused by variations of temperature and relative humidity and hence improvements have been and being required for protective layers.
On the other hand, since liquid crystal display devices are often operated for a long time, the polarizing plate is required to show a prolonged durability that makes it free from degradation of image quality of the LCD after a long-term use in an environment where both the temperature and the relative humidity vary significantly. Particularly, it is required to withstand the increased intensity of light emitted from the back light and the temperature rise due to the back light that are being observed as a result of the recent improvement in the luminance of liquid crystal display device.
In short, the polarizing plate is required to show a satisfactory durability under harsh conditions including high temperatures and high relative humidities as the scope of application of liquid crystal display device expands.
Proposals have been made to solve the problem of uneven display images after a long-term use under conditions where the temperature and the relative humidity vary significantly. They include those of using a film showing a low water vapor transmission rate of a protective film for a polarizing plate (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 10-101907) and a proposal of arranging a layer showing a low water vapor transmission rate on cellulose acetate (see, for example, JP-A Nos. 62-161103, 2001-215331, 2002-301788, 2002-90546, and 2001-343526).
However, none of the above-cited patent documents fully describe water vapor transmission rate. In other words, the above-cited proposals are not satisfactory for overcoming uneven displays that arise in environments where the temperature and the relative humidity vary remarkably.
Additionally, it has been established that the water vapor transmission rate range within which the problem of light leakage that arises when the display device is exposed to a high humidity condition is overcome is different from the water vapor transmission rate range within which the problem of light leakage that arises when the display device is exposed to a low humidity condition is overcome, and that when the water vapor transmission rate of the protective layer is overly reduced in an attempt to overcome the two light leakage problems at the same time, the load of drying process during the polarizing plate processing operation increases and thereby reduces productivity.